DC and high-frequency conductivity of the organic metals beta"-(BEDT-TTF)2SF5RSO3 (R = CH2CF2 and CHF)

The temperature dependences of the electric-transport properties of the two-dimensional organic conductors beta"-(BEDT-TTF)2SF5CH2CF2SO3, beta"-(d8-BEDT-TTF)2SF5CH2CF2SO3, and beta"-(BEDT-TTF)2SF5CHFSO3 are measured by dc methods in and perpendicular to the highly-conducting plane. Microwave measurements are performed at 24 and 33.5 GHz to probe the high-frequency behavior from room temperature down to 2 K. Superconductivity is observed in beta"-(BEDT-TTF)2SF5CH2CF2SO3 and its deuterated analogue. Although all the compounds remain metallic down to low-temperatures, they are close to a charge-order transition. This leads to deviations from a simple Drude behavior of the optical conductivity which become obvious already in the microwave range. In beta"-(BEDT-TTF)2SF5CH2CF2SO3, for instance, charge fluctuations cause an increase in microwave resistivity for T < 20 K which is not detected in dc measurements. beta"-(BEDT-TTF)2SF5CHFSO3 exhibits a simple metallic behavior at all frequencies. In the dc transport, however, we observe indications of localization in the perpendicular direction.Comment: 8 pages, 9 figure

Evolution of magnetic interactions in a pressure-induced Jahn-Teller driven magnetic dimensionality switch

We present the results of high-field magnetization and muon-spin relaxation measurements on the coordination polymer CuF 2 (H 2 O) 2 (pyrazine) in pressures up to 22.5 kbar. We observe a transition from a quasi-two-dimensional to a quasi-one-dimensional antiferromagnetic phase at 9.1 kbar, driven by a rotation of the Jahn-Teller axis. Long-range antiferromagnetic ordering is seen in both regimes, as well as a phase separation in the critical pressure region. The magnetic dimensionality switching as pressure is increased is accompanied by a halving of the primary magnetic exchange energy J and a fivefold decrease in the ordering temperature T N . J decreases gradually with pressure in the two-dimensional phase, and then increases in the one-dimensional regime. We relate both effects to the changes in the crystal structure with applied pressure

BEDT-TTF organic superconductors: the entangled role of phonons

We calculate the lattice phonons and the electron-phonon coupling of the organic superconductor \kappa-(BEDT-TTF)_2 I_3, reproducing all available experimental data connected to phonon dynamics. Low-frequency intra-molecular vibrations are strongly mixed to lattice phonons. Both acoustic and optical phonons are appreciably coupled to electrons through the modulation of the hopping integrals (e-LP coupling). By comparing the results relevant to superconducting \kappa- and \beta-(BEDT-TTF)_2 I_3, we show that electron-phonon coupling is fundamental to the pairing mechanism. Both e-LP and electron-molecular vibration (e-MV) coupling are essential to reproduce the critical temperatures. The e-LP coupling is stronger, but e-MV is instrumental to increase the average phonon frequency.Comment: 4 pages, including 4 figures. Published version, with Ref. 17 corrected after publicatio

Metal-insulator transition and charge ordering in the extended Hubbard model at one-quarter filling

We study with exact diagonalization the zero temperature properties of the quarter-filled extended Hubbard model on a square lattice. We find that increasing the ratio of the intersite Coulomb repulsion, $V$, to the band width drives the system from a metal to a charge ordered insulator. The evolution of the optical conductivity spectrum with increasing $V$ is compared to the observed optical conductivity of several layered molecular crystals with the theta and beta'' crystal structures.Comment: 5 pages, 3 figure

Images of interlayer Josephson vortices in single-layer cuprates

The interlayer penetration depth in layered superconductors may be determined from scanning Superconducting QUantum Interference Device (SQUID) microscope images of interlayer Josephson vortices. The authors compare their findings at 4 K for single crystals of the organic superconductor {kappa}-(BEDT-TTF){sub 2}Cu(NCS){sub 2} and three near-optimally doped cuprate superconductors: La{sub 2{minus}x}Sr{sub x}CuO{sub 4}, (Hg, Cu)Ba{sub 2}CuO{sub 4+{delta}}, and Tl{sub 2}Ba{sub 2}CuO{sub 6+{delta}}

Interlayer magnetoresistance in the organic superconductor {kappa}-(BEDT-TTF){sub 2}Cu[N(CN){sub 2}]Br near the superconducting transition

In this paper, we report transport measurements of interlayer magnetoresistance with field parallel and perpendicular to the current direction in the organic superconductor {kappa}-(BEDT-TTF){sub 2}Cu[N(CN){sub 2}]Br. For H{parallel}J, the isothermal magnetoresistance R(H) displays a peak effect as a function of field. While the magnetoresistance at small field can be fitted to stacked Josephson junction model, the negative magnetoresistance is not consistent with quasiparticle tunneling model with a simple mean field gap. The origin for the peak effect remains unresolved. For H{perpendicular}J, R(H) increases monotonically with increasing field. Large magnetoresistance for H{perpendicular}J is consistent with the layered structure of the organic compounds. {copyright} {ital 1999} {ital The American Physical Society

Rational design of organic superconductors through the use of the large, discrete molecular anions M(CF{sub 3}){sub 4}{sup -}(M = Cu, Ag, Au) and SO{sub 3}CF{sub 2}CH{sub 2}SF{sub 5}{sup -}

A new approach to synthesis of organic superconductors has recently been pioneered which involves the use of large discrete molecular anions as the charge-compensating entities in these charge transfer salts. The organic electron-donor molecule bis(ethylenedithio)tetrathiafulvalene (BEDT-TTF or ET) has been electrocrystallized with the novel organometallic M(CF{sub 3}){sub 4}{sup -} (M=Cu, Ag, Au) anions in a variety of 1,1,2-trihaloethane solvents. Over 20 organic superconductors have been synthesized which can be described by (ET){sub 2}M(CF{sub 3}){sub 4}(1,1,2- trihaloethane). These solvated salts are shown to have highly anisotropic physical properties which can be tuned via modifications of each of their three molecular components: ET electron donor molecule, M(CF{sub 3}){sub 4}{sup -} anion, and neutral 1,1,2- trihaloethane solvent molecule. Superconductivity has also been observed in an ET salt containing the discrete SF{sub 5}CH{sub 2}CF{sub 2}SO{sub 3}{sup -} anion with onset temperature near 5.2 K

Mixed-state magnetoresistance in organic superconductors {kappa}{endash}(BEDT-TTF){sub 2}Cu(NCS){sub 2}

We report transport measurements with field and current parallel to the {ital a} axis (perpendicular to the conducting plane) in the organic superconductor {kappa}{endash}(BEDT-TTF){sub 2}Cu(NCS){sub 2}. The magnetoresistance displays a peak effect as a function of field and temperature with the peak fields increasing linearly with decreasing temperatures. The peak resistance is found to be greater than the normal-state value extrapolated from both high- and low-field measurements. The results are in sharp contrast to the conventional dissipation mechanisms in the mixed state for anisotropic superconductors, as in the case of Bi{sub 2}Sr{sub 2}CaCu{sub 2}O{sub 8}. We propose a phenomenological model that the peak in the magnetoresistance is caused by a scattering mechanism due to a strong coupling to the underlying crystal lattice of fluctuating vortices (vortex polarons). The model can semiquantitatively fit the data. {copyright} {ital 1996 The American Physical Society.